Pumpers are portable pieces of equipment designed to deliver a cryogenic liquid such as nitrogen, for example, for temporary oilfield and industrial applications. The pumpers transfer nitrogen, for example, typically with a high-pressure positive-displacement pump, through an onboard vaporizer to a customer's piping, well, or other usage point. Pumpers utilize an onboard diesel engine to drive the pump and hydraulic pumps for ancillary circuits.
Nitrogen is delivered and stored in a cryogenic liquid state, and must be vaporized into the gaseous state and warmed for use in most applications. Many common materials become brittle, however, if exposed to cryogenic temperatures. Thus, the nitrogen must be warmed, prior to usage, to prevent unwanted failure or cracking. The original design of the pumpers utilized a direct-fired vaporizer to vaporize and warm the nitrogen.
Pumpers comprising direct-fired vaporizers include a forced-air liquid-fuel burner and a heat exchanger to transfer heat from the combustion gas into a nitrogen stream. The direct-fired vaporizers contact hot combustion gas directly to a high pressure tube bundle containing the cryogenic fluid.
A less common indirect-fired vaporizer may also be used in the pumpers. The less common indirect-fired vaporizers differ from direct-fired vaporizers in that an intermediate heat transfer fluid, typically a water-ethylene glycol stream, which is circulated to transfer heat from the combustion gas into a smaller high pressure heat exchanger tube bundle containing the cryogenic fluid, is used.
Both direct-fired vaporizers and indirect-fired vaporizers utilized in the pumpers are relatively simple and provide high heat exchange rates in a compact unit; however, both units are very fuel inefficient. Moreover, as a result of increasing fuel costs, both units have a very high relative operating cost. Finally, both units may not be utilized in some areas where open flame restrictions are in place.
For a variety of reasons, including, but not limited to, elimination of open flame conditions for work in locations with potentially flammable atmospheres and reduced fuel consumption, pumpers were adapted to use unfired vaporizers. A pumper incorporated with an unfired vaporizer, also referred to as a heat recovery pumper, loads its diesel engine above the power output required for the nitrogen high pressure positive displacement nitrogen pump and captures the heat from the engine coolant and the hydraulic system. Heat recovery pumpers that utilize a water-brake circuit to load the engine may also capture heat from that circuit as well. Often, the heat is also captured from the engine exhaust gas and engine turbo-charge air circuits, and sometimes other smaller heat sources as well. Heat recovery pumpers require a coolant circulation pump to circulate a water-ethylene glycol mixture to transfer heat from all the heat sources listed above into a coolant vaporizer, which houses the high pressure nitrogen heat exchange tube bundle inside a pressurized coolant vessel.
Heat recovery pumpers typically have better fuel efficiency than pumpers with a fired vaporizer, but for a given unit size, the heat recovery pumpers generally yield about half the nitrogen capacity of a direct-fired unit. Further, the heat recovery pumpers are limited to delivering nitrogen at discharge temperatures around 300° F. (149° C.) and at relatively low nitrogen delivery rates. In contrast, a direct-fired pumper is able to deliver nitrogen at high discharge rates or at temperatures around 600° F. (316° C.), which is desirable for certain industrial applications that use nitrogen as a heating medium.
As a result of the drawbacks of pumpers utilizing both fired and unfired vaporizers, the technology was combined. Fired and unfired vaporizer technologies were combined in parallel to form a single dual-mode pumper unit. The dual-mode pumper unit can utilize either the fired vaporizer or the unfired vaporizer at the discretion of the person operating the equipment. The unfired vaporizer is preferable due to its lower fuel consumption and necessary where the open-flame of the fired vaporizer is potentially a hazard, but the fired vaporizer may be used when the desired nitrogen discharge rate or temperature is beyond the capability of the unfired vaporizer.
Thus, there is a need in the art for a pumper unit that is more fuel efficient than conventional direct-fired vaporizers at all operating conditions, is able to provide high discharge temperatures up to 600° F. (316° C.), is able to discharge high flow rates up to 500,000 standard cubic feet per hour (14,158 nm3/hr) at ambient temperature, and is operated in an efficient manner.